1. Field of the Invention
The present invention relates to an FET amplifier for amplifying a high-frequency signal, particularly an FET amplifier controlled by a gate bias control signal, a pulse modulation module for pulse modulating a high-frequency signal by using the FET amplifier, and a radar device for detecting a target by transmitting a pulse modulation signal from the pulse modulation module to the outside.
2. Description of the Related Art
As a device for detecting the distance to a target of a second vehicle existing in front of a first vehicle, etc., and the speed of the target, various millimeter wave radar devices are utilized and, among them, there are radar devices using an amplitude pulse modulation system. In a millimeter wave radar device using an amplitude pulse modulation system, a pulse modulation signal is generated and radiated by changing the amplitude of a high-frequency signal of a fixed frequency by an ON/OFF controlled signal. Then, the millimeter wave radar device detects the distance to a target based on the time difference between the transmission signal and the reception signal by receiving a reflected signal from the target. In the millimeter wave radar device using such an amplitude pulse modulation system, an RF switch for making a high-frequency signal intermittent by using an ON/OFF control signal or an amplifier for controlling and amplifying an amplitude is required.
There is such an amplifier having the structure as described above in Japanese Unexamined Patent Application Publication No. 5-83041. Although the circuit in Japanese Unexamined Patent Application Publication No. 5-83041 is an amplifier for portable telephones, an amplifier used in millimeter wave radars also has substantially the same structure. This type of amplifier contains an FET, a stabilized negative voltage generation circuit, and a gate bias control circuit for generating a control signal of the FET. The gate bias control circuit contains a ROM in which the gate bias control data is stored, a D/A converter for converting a control data output from the ROM to an analog control signal, and an inverting amplifier in which an analog voltage from the D/A converter and a negative voltage from the stabilized negative voltage generation circuit are input to generate a negative gate bias voltage. Then, the FET is controlled in accordance with a gate bias control signal, which is controlled based on the intensity of a receiving signal or the command from the base station, and the output power is optimized by changing the amplification degree of a high-frequency signal input to the gate.
Furthermore, in Japanese Unexamined Patent Application Publication No. 11-195935, a circuit for obtaining a desired gate bias voltage by dividing a voltage using resistors is shown. In the circuit, a depletion-type FET is ON/OFF controlled by making the pinch-off voltage of the FET a threshold voltage of the gate bias voltage. Thus, a pulse modulated high-frequency signal is output.
When a millimeter-wave band signal is amplified using a depletion-type FET, the maximum output can be often obtained by setting the gate bias voltage of the FET about +0.2 V.
However, in the related amplifiers, since the gate bias voltage of the FET is always negative, it is not able to output a millimeter wave signal having a sufficient amplitude from the FET.
Furthermore, a stable gate bias voltage of a fixed amplitude is required in order that the FET in the ON state may perform a stable high output operation. However, in the related amplifiers, since the fixed gate bias voltage for making the FET in the ON state (making the FET have a large output amplitude) is generated by using the output voltage of a D/A converter, the gate bias voltage becomes unstable by the effect of variation of the power supply voltage of the D/A converter. Thus, the amplitude of a millimeter wave signal output from the FET also becomes unstable. Furthermore, as in Japanese Unexamined Patent Application Publication No. 11-195935, when a gate bias voltage is obtained by dividing a voltage by resistors, since the capacitance (capacitor) connected in parallel to the gate bias circuit and the resistors for voltage division constitute an integration circuit, a sharp waveform necessary for a high-speed pulse modulation cannot be obtained.